1. Field of the Invention
An aspect of the invention relates to an optical pickup apparatus and a controlling method thereof, and more particularly to an optical pickup apparatus capable of accurately detecting a kind of optical recording medium by compensating for a hysteresis phenomenon that occurs during movement of an actuator, thereby making the actuator move linearly, and a controlling method thereof.
2. Description of the Related Art
CDs, DVDs, and more recently, BDs (Blu-ray Discs) are widely used as optical recording media. Since a distance between tracks and a minimum pit length on the CD, DVD, and BD media are different for each type of medium, a wavelength of a laser used in recording and reproducing data and a data recording are also different for each type of medium.
An infrared laser having a wavelength of 790 nm is used for recording and reproducing data on and from a CD. A data recording capacity of a CD is 650 MB.
A DVD is made from two discs having the same thickness glued together to form two sides. A recording layer is formed on either one surface or both surfaces of either one side or both sides to form a DVD having from one to four recording layers that can be designed to be read from either one side (single-sided) or both sides (double-sided). A red laser having a wavelength of 635-650 nm is used for recording and reproducing data on and from a DVD. A single-layer DVD has a data recording capacity of up to 4.7 GB.
A BD, unlike a DVD, is made from one disc and is designed to be read from one side. A blue laser having a wavelength of 405 nm is used for recording and reproducing data on and from a BD. A maximum data recording capacity of a single-sided, single-layer BD is 25-28 GB.
Also, a distance from a surface of the medium to a recording layer is different for each type of medium. For example, the distance from the surface of the medium to the recording layer is approximately 1.2 mm for the CD, 0.6 mm for the DVD, and 0.1 mm for the BD.
An optical pickup apparatus is used to record or reproduce data on or from CD, DVD, and BD media. As shown in FIG. 1, the optical pickup apparatus includes a laser diode 10, an HWP (half-wave plate) 15, a beam splitter 20, a reflecting mirror 25, a CL (collimating lens) 30, an OL (objective lens) 40, an HOE (holographic optical element) 35, a photodetector 50 such as a PDIC (photodiode integrated circuit), an actuator 60, and a controller 65.
The laser diode 10 includes a plurality of light sources to generate laser light having at least two different wavelengths for use in recording or reproducing data on or from at least different two media selected from a CD, a DVD, and a BD. The HWP 15 rotates the polarization direction of the laser light from the laser diode 10 to be perpendicular to the plane of the figure. The beam splitter 20 reflects the laser light toward the reflecting mirror 25 through the CL 30 which collimates the laser light. The reflecting mirror 25 reflects the laser light toward a medium 70 through the HOE 35. The HOE 35 circularly polarizes the laser light, and the circularly polarized laser light is focused by the OL 40 on the medium 70.
Light reflected from the medium 70 passes through the OL 40 and the HOE 35 toward the reflecting mirror 25, which reflects the light toward the beamsplitter 20 through the CL 30. The light then passes through the beam splitter 20 and is incident on the photodetector 50.
The actuator moves the OL 40 in a direction along an optical axis of OL 40 toward or away from the medium 70 to perform a focusing operation, and moves the OL 40 in a direction perpendicular to tracks of the medium 70 to perform a tracking operation.
Because different wavelengths of laser light are used to record and reproduce data on and from a CD, a DVD, and a BD, it is important that an optical pickup apparatus is capable of detecting the kind of optical recording medium mounted on the optical pickup apparatus and generating an a laser light appropriate for use with the detected kind of optical recording medium. To this end, when the optical recording medium 70 is mounted on the optical pickup apparatus, the controller 65 controls the actuator 60 to move toward or away from the optical recording medium 70 until two pulses are generated in an output signal of the photodetector 50 while the actuator 60 is moving in one direction, either toward or away from the optical recording medium 70. One pulse is generated when a focal point of the light focused by the OL 40 moves onto the surface of the optical recording medium 70 and the focused light is reflected from the surface, and the other pulse is generated when the focal point passes moves onto the recording layer of the optical recording medium 70 that is closest to the surface of the optical recording medium 70 and the focused light is reflected from the recording layer.
Referring to FIG. 2, the magnitude of an input voltage applied by the controller 65 to the actuator 60 to drive the actuator 60 to move toward or away from the optical recording medium 70 varies linearly with time. It is desirable that the actuator 60 move at a constant speed, and accordingly the input voltage and time satisfy a linear function.
Assuming that the actuator 60 moves at a constant speed, a time interval between the two pulses in the output signal of the photodetector 50 will depend on the distance between the surface of the optical recording medium 70 and the recording layer of the optical recording medium 70 that is closest to the surface of the optical recording medium 70, with a greater distance producing a greater time interval. Thus, in an ideal case, a time interval between the two pulses will be least for a BD for which the distance is 0.1 mm, will be greater for a DVD for which the distance is 0.6 mm, and will be greatest for a CD for which the distance is 1.2 mm.
FIG. 3A shows a DVD and the two positions of the OL 40 at which the two pulses are generated. A distance between these two positions is 0.6 mm and corresponds to a time interval t1 between the two pulses as shown in FIG. 3A.
FIG. 3B shows a CD and the two positions of the OL 40 at which the two pulses are generated. A distance between these two positions is 1.2 mm and corresponds to a time interval t2 between the two pulses as shown in FIG. 3B.
However, it turns out that, contrary to the ideal case discussed above, the time interval t2 between the pulse corresponding to the surface of the CD and the pulse corresponding to the recording layer of the CD is actually smaller than the time interval t1 between the pulse corresponding to the surface of the DVD and the pulse corresponding to the recording layer of the DVD. This result is shown in FIG. 4.
As discussed above, in the optical pickup apparatus shown in FIG. 1, an input voltage applied by the controller 65 to the actuator 60 to drive the actuator 60 varies linearly with time. However, a hysteresis phenomenon occurs as shown in FIG. 5, in which an actual driving voltage forms a hysteresis curve when plotted against distance to obtain a motion curve of the actuator 60. The shape of a portion of the hysteresis curve obtained when the actuator 60 moves toward the optical recording medium 70 is different from the shape of a portion of the hysteresis curve obtained when the actuator 60 moves away from the optical recording medium 70. Furthermore, the actuator 60 does not move at a constant speed in response to a linear input voltage. Rather, the actuator 60 moves slower when the actuator 60 is very close to or very far from the optical recording medium 70, and moves faster when the actuator 60 is at intermediate distances from the optical recording medium 70. Thus, the motion curve of the actuator shown in FIG. 5 is a non-linear motion curve.
Since the driving speed of the actuator 60 varies depending on the hysteresis phenomenon, the time interval between the pulse corresponding to the surface of the optical recording medium 70 and the pulse corresponding to the recording layer of the optical recording medium 70 may be different from an expected time interval. That is, as shown in FIG. 4, in a case where the time interval t1 between the pulse corresponding to the surface of a DVD and the pulse corresponding to the recording layer of the DVD is increased as much as t′ from the ideal case (t1), and the time interval t2 between the pulse corresponding to the surface of a CD and the pulse corresponding to the recording layer of the CD reduced as much as t″ from the ideal case (t2), the controller 65 may confuse the CD with the DVD. If the controller 65 confuses the CD with the DVD and the laser diode 10 generates laser light having a wavelength of 650 nm for use with a DVD when a CD is actually mounted on the optical pickup apparatus, an error occurs and the optical pickup apparatus stops working.
Therefore, it is necessary to prevent the speed of the actuator from changing due to the hysteresis phenomenon, so that the actuator can be driven linearly.